Summary The Delta Helicopters Ltd. Bell204B helicopter (registrationC-GTNP, serial number2028) was conducting slinging operations in the Wadlin Lake, Alberta, area. At approximately 1420 mountain daylight time, as the helicopter was descending to position a load of seedlings, the pilot heard a scraping sound from the engine. The engine then lost power and the helicopter descended rapidly, resulting in a hard landing and substantial damage. The helicopter remained upright with the load attached to the longline. Another company pilot located the wreckage at approximately 1915 and found the pilot with serious, but non-life-threatening, injuries. The pilot was extracted from the site at 2145. Ce rapport est galement disponible en franais. Other Factual Information The helicopter had departed Wadlin Lake for Block106 with a load of tree seedlings in three cargo nets hooked onto a carousel at the end of a 150-foot longline. In the final stage of the approach at destination, when the helicopter was approximately 250feet above ground while descending and reducing forward speed, the pilot heard a scraping sound and noticed a vibration, and the engine lost power. During the ensuing rapid descent, the load contacted the ground at the intended landing site, and the helicopter continued to descend in the direction of flight until ground impact approximately 80feet beyond the load. The time of the occurrence was approximately 1420 mountain daylight time.1 Disassembly and examination of the engine (LycomingT53-11B, serial numberLE08253) revealed that two blades had separated from the power turbine disc and caused extensive damage to the other blades, the shrouds and the power turbine nozzle. Blade No.1 had fractured at the disc platform from a pre-existing fatigue crack that extended for approximately 50percent of the blade chord back from the leading edge. Blade No.52 had fractured approximately inch above the platform from a pre-existing fatigue crack; the fracture extended for approximately 35percent of the blade chord forward from the trailing edge. It could not be determined which blade had failed initially, but the second weakened blade failed from debris impact. Scanning electron microscope examination of the crack-initiation points did not reveal the cause of the cracks, as none of the usual indications such as mechanical damage, discontinuities, inclusions or corrosion was evident. All remaining blades on the turbine disc were examined, and no further cracks were found. Two of the remaining blades were sectioned to try to determine if an overheat situation may have been responsible for a metallurgical weakness due to changes in the alloy microstructure. However, the results were inconclusive due to a lack of comparison data from the manufacturer. Other hot-section components between the combustion chamber and the power turbine nozzle were undamaged and did not exhibit heat distress. The turbine disc (part number140-250-6, serial number4454) was new with an overhaul life of 2000hours when installed during the previous engine overhaul on 14June2000. Total engine time in service since the overhaul was 1284hours. The turbine blades were on condition, with no timed overhaul or replacement life, and no time-in-service history. The last major inspection was the engine mid-life hot-section inspection, which had been completed about 191hours prior to the occurrence on 13July2002, at 1092.7hours since overhaul. The current engine type certificate holder researched the original manufacturer's database and found one previous similar occurrence for the T53-11Bengine. In that occurrence, the stress rupture was induced by temperatures in excess of the design operational limits. Transport Canada's Service Difficulty Report database contained one reported occurrence since1995, and the airframe manufacturer's database listed two possible events since1965. T53-11 model engines were manufactured from 1963to1967. The load consisted of 130boxes of seedlings; the average weight of each box, from a 10-box sample, was approximately 28.5pounds (lb). With a weight reduction of one pound per box to allow for the weight of water from a light rain shower between the time of the occurrence and the weighing of the boxes, the weight of the load of seedlings was calculated to be approximately 3575lb. These weights were consistent with the tree nursery projected weight of 27to 33lb per box, depending on moisture content. The carousel weighed 80lb, and the weight of the nets and longline were estimated to be 120lb, for a total load weight of approximately 3775lb. The gross weight of the helicopter was calculated to be approximately 9680lb at take-off and 9480lb at the crash site. C-GTNP had been modified as per Bell Service Instruction204-3 to increase the maximum allowable gross weight with external load from 8500lb to 9500lb. Calculated out-of-ground-effect hover weight for the conditions was8600lb. Figure1. Height-velocity diagram Bell 204B (Bell Helicopter) Section 1 of the approved flight manual for the Bell 204B (BH204) contains a height-velocity (H-V)chart, which has a shaded area that was originally a flight limitation . . . critical in the event of an engine failure during take-off, landing or other operation near the surface (seeFigure1). Operation in this critical area reduces the possibility of a successful autorotative landing in the event of an engine failure. This H-Vchart has been appended as an operating limitation for Canadian-operated helicopters conducting external load operations, as an addendum (BHT-204-FM-CAN-O) to the flight manual. However, the information is relevant for all low-level operations. Records indicate that the pilot was certified and qualified for the flight, in accordance with existing regulations. His total flying time was approximately 13250hours, of which approximately 4500hours were on the BH204 series. His medical was valid to 05November2003, and his pilot proficiency check was valid to01November2003. During the investigation, weather information was obtained from Alberta Forestry Services observation towers in the vicinity. The report for the Wadlin Lookout Tower (17nautical miles [nm] northeast of the occurrence area) at 1200was as follows: sky condition 6/10covered, mid-level cloud, visibility 20nm, winds east at 4knots, temperature 14C, dew point 10C. The report for the Talbot Lookout Tower (17nm southeast of the occurrence area) at 1200was as follows: sky condition 1/10cumulonimbus, 2/10low cloud, visibility 20nm, winds southeast at 6knots, temperature 15C, dew point 13C. Weather was not considered a factor in this occurrence.